Set Short Slot Time subfield in Capability field, if ieee80211com.ic_caps

Set Short Slot Time subfield in Capability field, if ieee80211com.ic_caps
has IEEE80211_C_SHSLOT and the current channel is a 2GHz one, as according
to IEEE Std 802.11g-2003 subclause 7.3.1.4:

"STAs shall set the Short Slot Time subfield to 1 in transmitted Association
 Request and Reassociation Request MMPDUs when the MIB attribute
 dot11ShortSlotTimeOptionImplemented and dot11ShortSlotTimeOptionEnabled are
 true. ..."

For us "dot11ShortSlotTimeOptionImplemented" is IEEE80211_C_SHSLOT bit in
ieee80211com.ic_caps and dot11ShortSlotTimeOptionEnabled is always true.

In the original way, we will be counted as long slot time STA, if the BSS
that we are joining temporarily turns off short slot time, since some already
joined STAs are incapable of short slot time.  After our joining, even if all
long slot time STAs leave, BSS still can't use short slot time, because we
misinformed AP that we are incapable of short slot time in association request.